The fetus lives in an environment that is exceptionally thermostable but special situations may arise that challenge fetal thermoregulatory capability. These include the hyperthermia of maternal fever, the hypothermia of extreme cold exposure, and the variable, colder temperatures after birth, when temperature regulation may become critical for survival of the newborn. The mechanisms regulating fetal heat production and elimination in-utero remain largely unknown. We therefore propose three lines of investigation to better understand this aspect of physiology. We will 1) study fetal heat production using a new type of differential calorimetry; 2) measure the fraction of heat the fetus loses to the mother through various excretory pathways including the placenta, skin, cord, amniotic fluid, and uterine wall and; 3) explore the various thermoregulatory responses the fetus and newborn make to maintain a constant temperature. These responses include changes in heat production, heat conductance, and umbilical flow, that might stabilize temperature. Investigations will be performed in mid- and late pregnancy, using unanesthetized fetal sheep, and in newborn lambs. Experiments are designed togive new understanding of fetal heat production and temperature control, and how these change throughout gestation and in the newborn period.